Many small internal combustion engines are supplied with a combustible charge of air and fuel using a carburetor. A typical carburetor includes a body at least partially defining a liquid fuel chamber, an air and fuel mixing passage, and one or more fuel passages in communication between the fuel chamber and the air and fuel mixing passage. The fuel passages communicate with the mixing passage between an air inlet at an upstream end and an air-and-fuel mixture outlet at a downstream end. Typically, a choke valve is disposed in the air and fuel mixing passage near the upstream end to control a quantity of air flowing into the mixing passage during engine cold starting and warm up. A throttle valve is disposed in the air-and-fuel mixing passage near the downstream end to control a quantity or flow rate of the air-and-fuel mixture flowing out of the mixing passage to the operating engine.
In operation, engine pistons reciprocate and induce a pulsating vacuum through the carburetor such that air is pulled through the mixing passage toward the engine. This airflow induces a pressure differential in the carburetor, thereby causing liquid fuel to flow out of the fuel passages and into the air and fuel mixing passage where the fuel becomes mixed with air to create the air and fuel charge. The carburetor controls the combustible charge of air and fuel by controlling the flow of liquid fuel that becomes entrained in the mixing passage airflow, and by controlling the flow of air into the mixing passage and/or the flow of air-and-fuel mixture out of the mixing passage. More specifically, the carburetor may be manipulated to adjust an air-to-fuel (A/F) ratio in accord with varying engine requirements during engine startup, idle, steady-state operation, maximum power output, changes in load and altitude, and the like.
In one example, an operator may manually rotate the choke valve to a substantially closed or “choke-on” position such that vacuum in the mixing passage will be greater than when the choke valve is open. Thus, a greater quantity of fuel will be pulled into the mixing passage for a more fuel-rich A/F ratio for supply to the engine. In other words, the choke valve may limit the air flow rate through the mixing passage relative to the fuel flow rate to block or “choke” air flow through the air-and-fuel mixing passage.
In another example, some carburetors have startup systems that provide supplementary fuel when cranking a cold engine by opening a “start fuel” or supplementary fuel supply passage provided separately from a primary fuel supply passage. Such startup systems typically stop the supply of the supplementary fuel once the engine has been successfully started.